We propose to carry out investigations of threshold conditions for changes in biological systems produced by ultrasound. Experimental work on selected cellular, intracellular and macromolecular systems will be carried out in collaboration between physically and biologically trained scientists. Much emphasis will be placed on design of experiments according to available knowledge, and analysis of results to determine basic principles involved. Physical theory will be developed were required, and experiments on model systems carried out to test hypotheses. The broad aim is to develop methods for calculating parameters (frequency, amplitude-distribution or intensity-distribution, pulse regime, time of irradiation, etc.) of an ultrasonic field which lead to specified changes in given biological systems. We expect our work to be part of a wide-ranging effort involving investigators at a number of institutions. Our biophysical approach, using relatively simple biological units and models is meant to complement other work with more complex systems. BIBLIOGRAPHIC REFERENCES: W.L. Nyborg, D.L. MIller and A. Gershoy, Bioeffects at low ultrasonic levels in th neighborhood of gas-filled spaces. Paper delivered orally at Colloquium on Ultrasound Bioeffects and Dosimetry held at Chelsea College, London, on 22-23 July 1974. Abstract appears in Ultrasound in Med. & Biol. 1, 474 (1975). W.L. Nyborg, D.L. Miller and A. Gershoy, Physical consequences of ultrasound in plant tissues and other bio-systems, pp 277-299 in Fundamental and Applied Aspects of Nonionizing Radiation, published by Plenum Publishing Corp. (1976).